Contemporary pressure sensor applications require sensing pressures in a variety of pressure ranges from several kPa (kilo-Pascals) to thousands of kPa. Automotive applications include emission control, low pressure evaporative emissions, exhaust gas recirculation, intake manifold pressure, and fuel systems. A single sensor cannot effectively serve all of these applications because of the wide pressure ranges.
One type of pressure sensor positions a Piezo-Resistive Transducer (PRT) or silicon strain gauge onto a relatively thin diaphragm. When the diaphragm is subjected to varying pressures the diaphragm deflects which causes the strain gauge to output a signal indicative of the varying diaphragm deflections. The cross sectional thickness of the diaphragm and the overall area of the diaphragm define its flexibility, or its tendency to deflect based on a change in pressure. So when subjected to a certain pressure a relatively thin diaphragm will exhibit a relatively large deflection, and a relatively large surface area diaphragm will also exhibit a relatively large deflection. The thinner the diaphragm the harder it is to fabricate accurately with high production yields, and the larger the surface area of the diaphragm the more costly and bulky the sensor becomes. As a practical matter to accurately sense relatively low pressures in certain low pressure applications the diaphragm must be relatively thin. Consistently fabricating thin diaphragms is difficult and time consuming. For a relatively thin diaphragm small diaphragm thickness deviations from a nominal are greatly magnified since a fixed tolerance becomes a larger portion of the diaphragm thickness. This in turn creates large variations in the sensing element span and pressure non-linearity, variations in two parameters which play a major part in determining any sensing element quality.
At present, to cover all the required pressure ranges, custom silicon PRT pressure sensing elements are built for each specific pressure range. These custom sensing elements are implemented with different diaphragm areas and diaphragm thicknesses, and of the different sizes. This makes manufacturing complex. Moreover relatively thin and relatively large surface area diaphragms will cost considerably more to manufacture than thicker and smaller diaphragms because of material cost and manufacturing process complexity.
In current PRT sensors, to meet the sensitivity requirements of low pressure applications a relatively large surface area diaphragm is required. In a monolithic sensor design, which integrates sensing element and electronics on the same die, this relatively large surface area diaphragm leaves less area around the die for a signal conditioning circuit. It creates very inefficient or almost impossible Integrated Circuit (IC) layout of such monolithic die. Moreover a new and different layout has to be made for each substantially different pressure range which is expensive and time consuming. In addition, it is difficult to use timed etch technique to etch the diaphragm because any single type of sensing element can only tolerate a small diaphragm thickness deviation from the nominal. The problem is greatly magnified for sensors where the diaphragm has to have a relatively large surface area and also relatively thin, since a fixed tolerance becomes larger portion of the diaphragm thickness which in turn creates large variations in the sensing element span and pressure non-linearity.
What is needed is an improved pressure sensor that can accurately operate over a wide pressure range that is easier to manufacture.